How to speed up when blender has suddenly slowed down?
Yes, this is normal.  The fact that the CPU utilization was so low tells me that your daughter has probably 'baked'[1] the simulation to a disk cache so most of the 'work' will be showing up in the form of disk I/O rather than CPU utilization.  This is a good thing and probably giving her much better performance than if she 'frees' the bake.  The physical sims (smoke, fire, water, soft body) are fun but expensive: they are quite computationally and storage intensive and will often soak up any amount of performance a consumer grade system has to offer.  The performance and storage issues get exponentially worse as you dial up the detail, such as going from 32 to 256 divisions.

Some tips to help her deal with this:


When you're working out a scene, keep the detail settings of the sim as low as possible to provide the general 'feel' for how it's going to look when it's done.  Bake it for performance and work through as much as possible of the rest of the scene.
Once you have everything else worked out, free the bake
Now then start dialing up the detail level of the sim until it is just above her threshold of 'that looks the way I want it'
Bake the sim again at the final detail settings
Fine tune / finish up
Render the final animation


[1] To determine whether the smoke sim has been baked or not, select the smoke domain and then go to the 'Physics' tab of the Properties browser.  At the bottom there is a section 'Smoke Cache' and a set of 6 buttons in a group.  If the first button is labeled 'Bake', then she hasn't baked the sim.  If it is labeled 'Free Bake', then she has.  Either way, the smoke cache defaults to using a sub-directory in the same directory as the main .blend file (so if she has saved it as my.blend, there should also be a blendcache_my-smoke directory)